The FT-IR spectrophotometer consists of two basic parts: (1) an optical system which includes the interferometer and (2) a dedicated computer used to analyze the information contained in the light beam produced. The advantages and improved performance of a Fourier transform infrared (FT-IR) spectrophotometer result from the use of an interferometer, rather than a grating or prism, to obtain spectral data (FT-IR). An interferometer permits measurement of the entire spectral range of a sample in a fraction of the time previously required.
The operation of a Michelson interferometer to analyze infrared light which passes through a sample, as applied to FT-IR spectrophotometry, is well known. The interferometer consists of a pair of perpendicularly arranged optical paths, each having a reflector or mirror positioned at its end to reflect light traversing the path. One mirror is fixed. The other mirror is longitudinally movable to increase or decrease the length of the light path. A light beam entering the interferometer is split into two components by a beam splitter so that a separate component of the beam will traverse each optical path. After reflection the components are recombined at the beam splitter to constructively and destructively interfere. The reconstructed beam is thereafter directed through a sample and focused onto a photodetector for measurement of intensity.
The intensity of the reconstructed wave depends on the difference in length of the optical paths over which the component beams travel. Generally, when the movable mirror is scanned at a constant velocity, the intensity of an emerging light beam will modulate in a regular sinusoidal manner for any selected wavelength of light passing through the interferometer.
A typical infrared light beam leaving the interferometer is a complex mixture of modulation frequencies due to its polychromatic nature. After the infrared light beam has passed through a sample material, it can be detected to determine those wavelengths of light which have been absorbed by the sample. This is accomplished by measuring change in the sinusoidal pattern expected when the light beam exits the interferometer. Measurement of the differences in the characteristic sinusoidal pattern for each light wavelength indicates those wavelengths of light which are absorbed by the sample. Infrared light absorbance characteristics provide a spectrum from which the material comprising the sample can be determined.
The output of a detector measuring the intensity modulation of the emerging beam can be recorded at very precise intervals during a mirror scan, to produce a plot known as an interferogram. The interferogram is a record of the output signal produced by the infrared detector as a function of the different length optical paths traversed by the components of the infrared beam in the interferometer. Successive measurements of the sample are obtained and co-added to obtain an average interferogram having improved signal-to-noise characteristics. The average interferogram provides information and data relating to the spectral characteristics of the sample material. After mathematical preparation, a Fourier transform calculation is performed on the interferogram to obtain a spectral fingerprint of the sample composition. The results are compared against known reference data to determine the composition of the sample.
Most Fourier transform techniques require averaging of a large number of interferograms in order to obtain accurate results. As many as 32 to 50 scans during which measurements are taken may be averaged. It is important for an interferogram to be precisely reproducible in order to maintain accuracy in their averaging. Since an interferogram is created by measurement intensity modulation, more accuracy in the interferogram and resultant Fourier transformation will also result if more accuracy is obtained in the measurement of intensity throughout the time during which data points are measured to define the interferogram.
To accomplish accuracy and reproducibility for an interferogram, an adjustable mirror bounding the fixed path of the interferometer must be maintained in optimal alignment with the beam splitter and the moving mirror. This is most often accomplished by providing a biaxial adjustment for the fixed mirror so that it may be adjusted about two axes to bring the image of its surface into absolute parallel alignment with the reflecting surface of the moving mirror. It is extremely important that the image of the adjustable mirror be maintained parallel to the reflecting surface of the moving mirror. Parallelism must be maintained within one wavelength of the shortest wavelength of light being measured to generate the interferogram. Failure to provide precise alignment results in reduction of the magnitude of identifying peaks in the interferogram produced, reduction in signal-to-noise performance and phase error introduction in the heterodyne beam leaving the interferometer. Each of these substantially reduces the accuracy with which an interferogram can be reproduced and the precision with which the interferogram can be analyzed to determine the sample composition. The accuracy with which a spectrophotometer measures intensity modulation can be no better than the limited precision of its components. The molecular geometries cannot be accurately determined if the modulation frequencies are not reproduced and measured precisely.
Prior art designs have used massive structures and extremely fine mechanical adjustments to obtain accurate mirror alignment. Temperature compensation has also been attempted to reduce thermal distortion error. However, it has been found that static alignment cannot assure accurate mirror alignment throughout a scan of a moving mirror. Wobble and support inaccuracies of the moving mirror continue to introduce alignment error. Dynamic mirror misalignment results in aperiodic errors in the intensity measurements obtained, which generate unpredictable and accuracy-reducing gliches in the interferogram profile.
Modern systems accomplish mirror alignment automatically by passing a reference light beam, such as a laser beam, concurrently through the interferometer with the infrared light. The reference beam is used to directly measure misalignment between the fixed and movable mirrors.
Laser beams have been used most effectively. Since a laser beam undergoes the same splitting and traverse of changing optical paths as the infrared light in the interferometer, the recombined laser beam exhibits a measurable monochromatic wavelength having an interference pattern containing information indicative of mirror alignment. A phase difference measurement may be obtained across the width of the laser beam to determine the difference in the length of the path traversed by one portion of the beam relative to another. Unequal phase measurements are indicative of unequal path lengths indicating mirror misalignment.
In a conventional system, when the movable mirror is moving at a constant velocity, a doppler shift is generated in the component of the laser beam traversing the changing optical path. When the doppler shifted beam is recombined with the component traversing the fixed length path, a modulated frequency beam exhibiting measurable beat frequency is produced. The recombined beam yields a series of varying intensity or fringe patterns which may be analyzed across the cross section of the beam to determine mirror alignment. Conventional systems generally drive the moving mirror at a velocity which produces a 5 KHz modulation, i.e., doppler shift, in the exiting beam. At faster mirror velocities, the modulation will increase providing increased resolution for alignment measurement, while at slower mirror velocities the modulation will decrease. Precision with this technique can be maintained to approximately one cycle in 5,000.
In a conventional system, however, a movable mirror must be scanning to obtain a doppler shift in the light beam traversing its path, and thus a measurable modulation signal. When the movable mirror is stationary, the light beams traveling along adjacent paths of the interferometer are combined to form an identical frequency light beam without modulation. Thus when the mirror is not moving, there is no information obtained in the recombined beam which can be used to determine mirror alignment. This occurs at every instance that the movable mirror reaches the end of its scan and stops before turning around to proceed in the other direction. With prior art auto-alignment systems mirror alignment is lost at the ends of mirror scan.
Furthermore, with conventional systems modulation of the recombined beam becomes very difficult to measure as the velocity of a mirror scan becomes very slow. For instance, for a 0.3 centimeter per second scan velocity, a modulation frequency of 5 KHz is obtained in the recombined light beam. However, if the mirror is driven at a scan velocity of 0.03 centimeters per second, the modulation frequency is reduced to 0.5 KHz. Thus, as the scan velocity is decreased, the modulation frequency in the recombined light beam decreased to a level which is difficult to measure with modern electronic detectors, providing no control of alignment.
An FT-IR spectrophotometer has limited resolution for frequency measurement determined by its limited ability to produce an interferogram. The the optical system is fundamental in determining the accuracy with which a spectrophotometer can measure frequencies. The accuracy with which the spectrophotometer can analyze a sample is directly related to the ability of the instrument to produce an accurate measure of intensity of the emerging infrared beam. This requires proper alignment of the fixed and movable mirrors.
Conventional use of a laser reference to obtain mirror alignment continues to suffer limited precision and control ability. Improvements in the precision with which mirror alignment can be measured and controlled will necessarily produce significant improvement in the accuracy which an FT-IR spectrophotometer can analyze a sample substance.